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1 STRENGTH TEST ON BOLTED CONNECTIONS USING HIGH-STRENGTH STEELS (HSS-STEELS) AS A BASE MATERIAL Jouko Kouhi Technical Research Centre of Finland Laboratory of Structural Engineering Kemistintie 3 SF ESPOO FINLAND Rakenteiden Mekaniikka, Vol. 25 No , ss RESUME The results of a series of tests on bolted connections using high-strength steel (HSS) as a base material are presented and analysed in this report. The base material used was HSF 640 (ISO 5951 ) which has a yield stress of 640 MPa. The tests studied bearing resistance, net section failure and the block shear failure mode. The test results have been analysed according to the Finnish specification for steel structures B7, the swedish specification BSK, the NKB-specification and according to the Eurocode 3. The block shear failure mode has been analysed according to the American LRFD-specification. The aim of this study has been to prov~de experimental test results to allow the development of reliable calculation methods for connections using HSS steels in different types of steel structures. INTRODUCTION The Finnish specification B7 for steel structures is not valid for the design of steel structures made of high strength steels (HSS steels). HSS steel means here steel with a yield point greater than 400 MPa. Some codes like the Swedish BSK, the Canadian CSA-Standard and the American LRFD-code also give rules for structures made of HSS steels but HSS steels are not yet generally accepted in codes. The highest steel grade in the Eurocode 3/Annex D is FeE 460. BEARING RESISTANCE OF BOLTED CONNECTIONS Figure 1 shows the principal arrangements used in the tests. Measures are also shown in Figure 1. There were two bolts in the line in test series E and four bolts in two lines in test series F. The variation of distances between bolts is given in the figure 1. The ejd- values for test series F given in parentheses are measured on the other side of the steelplate. 41

2 plate 2 ve, vp!" 1 '1 '1 plate 1 Test E1 E2 E3 E4 E5 E6 elf do 1, 20 1,20 1,19 1,50 1, 49 1,50 : I P1/ do + + +e;-+ I:: I 2,20 bolts M24, ,78 d 24 nun (the diameter 2,98 d 0 = 26 nun (the diameter 2, 18 er/do = 5, b 200 mm 2, 79 p ate 1, t 4 mm 2,99 plate 2, t 8 : of of the the bolt) hole) plate 2 "' e, '(" ~' p, "' 'I plate 1 F/ 2 F~ "' "' " ej " I:: I + + e2 i F/ 2 Test e1/do P1/do Fl F2 F3 F4 F5 F6 (1,75) 1,36 2,60 e2/do = 1,5 (do (1,75 ) 1,35 3, 3 0 P2/do = 6, 0 (do 1,38 2, 59 e3fdo 5, 9 1, 36 3,53 plate 1, t 3 mm 1, 77 3,30 plate 2 1 t 6 mm (1,77) 1, 40 3, 31 ( ) mm) mm ) Figure 1. Test for the bearing resistance, principal sketch and measured values, test series E and F. 42

3 A typical load- deformation curve for all the tests is shown in Figure 2, test no El. The notations "87/220/lowest'' in Figure 2 mean that the loadbearing resistance according to the Finnish code b7 is 220 kn. This means that the loadbearing resistance (2 20 kn) is calculated based on the lowest value of the loadbearing resistances of individual bolts. The notation "87/357/sum" means that the total loadbearing resistance of the connection is 357 kn, which is calculated by summarizing the loadbearing resistances of the individual bolts. Corresponding notations are used in Figure 2, when the calculations have been made according to the Eurocode 3 or the Swedish 8SK-regulations. Failure load 'DO ~00 Load ( kn) ~OD 300 For EC l/369/sum. all load-bearing resistances of individual bolts have been summarized, For EC J/317/lowest the load-bearing resistances of the whole connection have been calculated based on the lowest load-bearing resistance of individual bolts BKS/1.93/sum.. EC 3/369/ sum "-... EC3 /317/Iowest BSK /301./lowest""- El o;~plac em nt (1!1!1) 30 Figure 2. Typical load-deformation curve, tests El. CALCULATION METHODS Design according to the Finnish 87 specification The Finnish 87 specification (National Building Code of Finland) specifies FRh = k 2 d o t where o fy/1m ( 1) e 1 /d - Oo5 S 2o5 or pl/d - Oo5 s 2.5 ( 2) 43

4 FRh for the whole connection have been calculated on the basis the following assumptions a ) with the limitation k 2 ~ 2.5 and using the lowest bearing resistance value for the bolts b ) summarizing the bearing resistances of individual bolts and with the limitation k 2 ~ 2.5 (not allowed under the B7 code) c ) summarizing the bearing resistances of individual bolts without the limitation of k 2 (not allowed under the B7 code). Resistance of the gross-section (Agrl is NRy = Agr fy/1m ( 3 ) Resistance of the net-section (An ) is d is the nominal diameter of the bolt. If the shear plane intersects the threads or thread run-out, the diameter corresponding to the stress area A of the bolt is used for design purposes. Bolts with full tfireads were used in these tests. The d-value used in the calculation always corresponds to A (e.g. d = ds ), where ds has been calculated accoraing to the As values. In these calculations 1m = 1.0 is used. Design according to Eurocode 3 (EC 3 ) Bearing resistance EC 3 is determined from equation (5 ) under ( 4 ) FRh = FbRd = (2.5 a fu d t ) /1m 2 ( 5 ) where a is the smallest of 1.0 e 1 /3d 0 p 1 /3d ( 6 ) fublfu (the ultimate strength of the bolt/ the base mater~al ) d 0 d is the the ( in the comparative calculation, 1M 2 = 1.00 is used because only the calculation methods have been compared to the test result calculations) diameter of the hole and diameter of the bolt. The diameter of the hole and the diameter of the bolt have to be taken into account in calculations made according to EC 3. However, EC 3 gives no rules on which diameters should be used if the threads are in the shear plane. In these tests the shear plane passes through the threaded portion of the bolt. 44

5 The comparative calculations have been made using two methods: 1} Using the value ds based on the stress area in calculating a a = e 1 j3d 0 = e 1 j3ds or a = p 1 /3d = p 1 J3ds ) Using the hole diameter d 0 in calculating a a = e 1 /3d 0 or a = p 1 /3d Method 2 ) is given in EC 3. Method 1) is not given in EC 3, and is used here only for comparison. Design according to the Swedish CBSK) specification According to BSK regulations bearing resistance is calculated as FRh = FRbd = 1.2 (e 1 ;d ) d t fud ( 7) where d is the diameter of the bolt fbuk/1. 2 ( 8) the design value for the material and the characteristic value for material (ultimate strength). The coefficient 1.2 in equation (8} is a safety factor which has not been taken into account in the calculations below, which used the measured value fu. Design according to the Scandinavian CNKBl recommendations Bearing resistance according to the NKB guidelines is the same as in BSK, except that the coefficient in equation (7) is 1. 4 instead of 1.2. The results calculated by the different methods are compared with the test results in tables la and lb. 45

6 Table 1a. Calculated results compared to test results, series E. Specimen E1 E2 E3 E4 E5 Test load ( kn ) (Ntest> B7, lowest a ) value (kn ) 87 /Ntest B7, summation b ) k2 $ 2.5 ( kn ) 87 /Ntest B7 I summation c ) without limitation of k 2 (kn ) 87 / Ntest BSK, lowest a ) value (kn ) BSK/ Ntest BSK, summation b ) ( kn ) BSK/Ntest EC 3, lowest a ) method 1 ) value (kn ) method 2 ) EC 3 / Ntest method 1 ) method 2 ) EC 3, summation method 1 ) (kn ) b ) method 2 ) E EC 3/Ntest method 1 ) method 2 )

7 Table lb. Calculated results compared to test results, series F. Specimen Fl F2 F3 F4 F5 Test load (k.n) (Ntestl 87, lowest a ) value (k.n) 87 /Ntest , summation b) (kn) k2 :$ 87 /Ntest , summation c ) without limitation of k 2 (k.n) 87 /Ntest SK, lowest a) value (k.n) 8SK/Ntest SK, summation b) (k.n) 8SK/Ntest EC 3, lowest a) method 1) value (k.n) method 2 ) EC 3 /Ntest method 1) method 2) EC 3' summation method 1 ) (k.n) b) method 2 ) EC 3/Ntest method 1) method 2) F The following conclusions from the results in Table 1 can be made: - the code 87 gives results, which are very much on the safe side (calculation method a)) by summarizing all bearing resistances of individual bolts and ignoring the limitation k 2 $ 2.5 we get results on the safe side from Figure 2 it can be seen that the deformations at the ultimate limit state are quite large (about the diameter of the bolt). The large deformation together with safety requirements for bolted connections are actually the reasons for the calculation method (k 2 $ 2.5) given in code

8 - EC 3 rules gives results on the safe side - rules according to the BSK (summation) give results which are on the unsafe side in some cases. Generally BSK-rules seem to estimate quite well the test results (additional safety-factor 1.2 is not used in these comparisons). Block shear failure mode The block shear failure mode is illustrated in Figure 3. lin e 1 Figure 3. Block shear failure mode. The dimensions for the block shear failure mode tests are given in Figure 4. The values given in parentheses are measured on the opposite side of the connections. -+ : "'e, '.!: p, plate 1 "' I +- I +- ~ plate 2 i I + + I + + e2 I ~: I i Test e1/do P1/do Gl ( 1177) 1,41 3,54 e 2 ;d 1,5 do 26 mm G2 1,41 2,60 p 2 /d 2,4 d 24 mm G3 (1,77) 1,42 3,54 e 3 ;d 5,9 G4 1,75 2,60 plate 1, t 3 mm G5 (1,75) 1,42 3,30 plate 21 t 6 mm G6 {1,77) 1,40 3,31 Figure 4. Block shear failure mode, test series G, measured values. 48

9 According to the American LRFD- code (Load and Resistance Fact or Design) the loadbearing resistance of the connection is calculated using equations (9) and (10). Fblockl cp (0.6. fy. Avg+ fu. Antl (9) Fblock2 cp (0.6. fu. Ans + fy. Atgl (10) where cp 0.75 (cp 1.0 is used here for an analysis of the test results Avg is the gross area subjected to shear, Atg the gross area subjected to tension, Ans the net area subjected to shear and Ant the net area subjected to tension. The controlling equation is one that produces the larger force. shear area Failure by tearing out of shaded portion shear area area, small tension force Figure 5. Block shear failure mode, LRFD- specification. Under EC 3 the design ultimate shear resistance Vu R for rupture along a block shear failure path is taken as: (11) where Av.net is the net area subjected to block shear. - 49

10 The net area Av et subjected to block shear is determined, as indicated in P1gure 6, as follows: (12) in which L 1 and L 2 are given by: L 1 = 2.5d 0 but L 1 ~ a 1 L 2 = 5. 0d 0 but L 2 ~ a 2 (13) where n is the number of fastener holes on the block shear failure path. block shear failure path L1 T l..al Figure 6. Net shear area- block shear, Eurocode 3. The results of the comparison are given in Table 2. Table 2. Comparison of the test results with the calculated values, block shear failure mode. Test F1/Ftast I F~F5est NR/Ft st Accor 1ng to F -code Accora1ng to EC 3 Gl G G G G G

11 The results in Table 2 show that the LRFD-code gives quite good results ar.d the calculation method in EC 3 gives results on the safe side. It s hould be mentioned that in the comparison given in the table the material factor is 1.00 (1m 2 = 1.0 and~ = 1.0). Failure at the net section The test pieces in test series H were designed so that the failure would occur in the net section. The failure loads have been calculated using the equation (14) Nu = fu Anet ( 14 ) The ratio Ntesti Nu is given in Table 3. Table 3. Test series H, the failure at the net section. Test Ntesti Nu H H H H H H For design poposes EC 3 gives the formula ( 15) Nu = 0.9 fu. Anet11m2 (15) where 1m 2 = 1.25 (as general for bolted connections). CONCLUSIONS The bearing resistance ( 18 test specimens) and block shear failure ( 6 test specimens) of various bolted connections were tested and analysed. The base material was highstrength steel (according to ISO 5951 ) with a nominal yield stress of 640 MPa. The connections had two shear planes. The results of the bearing resistance tests were compared to the analytical values calculated by using the methods given in the Finnish, swedish and Scandinavian codes and Eurocode 3. The results of the block shear tes.ts were compar ed to the analytical values calculated using the American LRFD specification. The following conclusions can be made from the bearing resistance tests: - All the calculation methods used here give results on the safe side, if the bearing resistance of the whole 51

12 connection is calculated using the lowest bearing resistance of individual bolts. If the bearing resistance of the whole connection is calculated by summarizing the bearing resistances of each individual bolt, the Finnish B7 code and Eurocode 3 give results on the safe side while the Swedish BSK code seems to overestimate the bearing resistance of the whole connection if the additional safety factor of 1.2 (BSK ) is not taken into account. - If the bearing resistance of the whole connection is calculated by summarizing the bearing resistances of each individual bolt, the deformations at the ultimate limit state can be quite large (the magnitude of the diameter of the bolt). - Because of the different load- deformation behaviour of individual bolts (depending on the proportional end distance (e/d) and the proportional pitch (p/d)), it is recommended here that the bearing resistance of the whole connection should be calculated using the lowest individual bearing resistance. This will prevent the deformations from being excessive. The calculation method for block shear failure given in the American LRFD specification seems to give satisfactory results for design purposes. The method for block shear failure mode given in EC 3 seems to underestimate loadbearing capacity. The ultimate load at the net section can be calculated based on the ultimate tensile strength and the net section to an accuracy of approx. 5 %. The calculation methods given in the Finnish B7 code for steel structures can be used for HSS steels but the results produced seem to be on the safe side. Regulations for block shear failure should be added to the code. Statistical analysis based on additional test results on HSS steels needs to be made if more exact calculation methods are to be produced. REFERENCES 1. Bestammelser for stalkonstruktioner BSK. (The Swedish regulations for steel structures. ) Stockholm 1987, Statens Planverk. 140 p. 2. Eurocode 3. Design of steel structures. Volume 1. Chapters l - 9. Edited draft, Issue 2. Brussels 1989, Commission of the European Communities. 102 p. 3. Load and resistance factor design. Chicago 1986, American Institute of Steel Constructions. 219 p. 52

13 4. Nordic guide- lines for steel structures. 1982, The Nordic Committee for Building Regulations, NKB Report p. 5. Terasrakenteet. (The Finnish B7 Building Code, Steel structures. ) Helsinki 1988, Yymparistoministerio, Suomen rakentamismaarayskokoelma, Osa B7L 40 p. 6. Kouhi, J. Tests on single shear plane bolted connections made of high strength steel (HSS steel ). (The report is under preparation and will be published in ) 7. Kouhi, J., Kortesmaa, M., Strength tests on bolted connections using high-strength steels (HSS steels) as a base material. Espoo Technical Research Centre of Finland, Research Notes